Synergistic effects in binary systems of lubricant additives: a chemical hardness approach

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), f riction modifier molybdenum dithiocarbamate (Modtc) and detergent overbased calcium borate (CB) lubricant additives have been investigated by coupling analytical TEM and micro-spot XPS in the tribotester Optimol of SRV GmbH ( mild wear conditions in boundary lubrication). Synergistic effects have bee n observed on both friction and wear data, especially in the Modtc/Zndtp co mbination. Results have been interpreted on the basis of a chemical hardnes s concept: the hard and soft acids and bases (HSAB) principle, stabilisatio n of hard-hard pairs and the maximum hardness principle. The performance of the Modtc/Zndtp mixture is mainly due to the generation of MoS2 single she ets and the digestion of MoO3, which is also formed, by the zinc polyphosph ate glass. The final result of the tribochemical reaction is a tribofilm co mposed of MoS2 sheets embedded in a mixed Mo/Zn polyphosphate glass. The CB /Modtc mixture has a similar mechanism except that the oxide is not complet ely eliminated, due to the softer borate anion compared with the phosphate one.